The efforts which have extended over a long period and which have culminated in the present invention were aimed at providing a photo-detector signal amplifier useful in focal plane mosaic arrays. The problem is to provide satisfactory pre-amplification, at the focal plane, of each individual detector signal from the densely-packed infrared photo-detectors in a mosaic array.
The extremely high density requirement and the operating environment of such focal plane photo-detector arrays have created a problem for researchers in this field, which has only been satisfactorily resolved by the present invention. Among the very stringent limiting factors are the following:
(1) Space limitations. The amount of "real estate" available for pre-amplifiers receiving the individual detector signals should be as small as possible; and a value of approximately 800 square mils or less for each preamplifier is considered the required area specification.
(2) Low power limitations. Partly because of the extremely high density, and partly because of the operating environment, which utilizes cryogenic temperatures (less than 120.degree. K.) for detector efficiency, the power dissipation of the pre-amplifier must be kept extremely low.
(3) High gain requirements. In spite of the foregoing limitations, each pre-amplifier must supply a very high gain because the signal sensed by the detector is extremely weak. Since the available space is so drastically limited, the number of transistors in the pre-amplifier is severely restricted.
(4) DC bias requirements. It is very important that the DC bias point of the detector input to the pre-amplifier have both (a) a very "low" value (below 10 millivolts), and (b) a very stable value. In other words, the DC bias point at the pre-amplifier input should be held at a substantially constant value, within a range of a few millivolts from the detector reference voltage (which conveniently is ground). Without this DC bias point "low" value and stability, noise tends to be excessive, dynamic range is unduly limited, and bandwidth is unduly limited.
(5) Low noise requirements. The background are created by the detector's environment, and the noise created by the detector itself, are unavoidable. But it is important that any noise created within the pre-amplifier be less than the sum of the unavoidable noises, in order to maximize the sensitivity of the system. Once the pre-amplifier noise is reduced below that sum, no further noise reduction in the pre-amplifier is necessary.
(6) Variable gain adjustability. It is highly desirable to provide a pre-amplifier whose gain can be adjusted to adapt to different operating situations.
The focal plane electronic systems proposed prior to, or concurrently with, the present invention have been deficient in most of the aspects discussed above.
Generally, the efforts to solve the problem have relied on charge coupled devices (CCDs), in which a stored charge at the semiconductor surface can be made to propagate along the surface via potential wells created by a series of MOS structures. Amplifications in CCDs is accomplished by "pumping" the charge into successively smaller wells.
Where pre-amplification transistors have been tried in focal plane systems, they have been single transistor units intended to provide sufficient gain to introduce the signal into a subsequent amplification stage, such as CCDs.
While such efforts may have met the space limitations and low power dissipation requirements stated above, they have not provided low and stable DC bias, high amplifier gain, low noise, or variable gain.
In other environments than the extremely demanding focal plane electronics package under consideration, the use of operational amplifiers might have been considered, because of their known virtues. However, the only technology suitable for the low temperature environment is MOSFET technology, which in a multi-transistor system was considered to require power dissipation far beyond the limits permissible in focal plane amplification. This essentially universal assumption of others working in this field has proved to be a major error; but it may have resulted from the predominance of digital technology as the field of use of MOSFET devices, and particularly of CMOS integrated circuitry.